The invention relates to a method of compounding heat-vulcanizable silicone compositions.
A heat-vulcanizable silicone composition comprises a high viscosity silicone polymer, an inorganic reinforcing filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent can be added and the composition heat-cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
Typically, the heat-vulcanizable silicone composition is produced by kneading a high-viscosity polydiorganosiloxane, the inorganic filler and additives by means of a batch kneading machine such as a high intensity Banbury mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler and treating agents are batch mixed until desired properties are obtained. This process requires long residence times and large amounts of energy. Non-homogeneous shear and extensional stresses across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
In Kasahara et al., U.S. Pat. No. 5,198,171, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed by a high speed mechanical shearing mixer. The resulting premix is further compounded in a same-direction double screw extruder. The premix is formed in a first step wherein a diorganopolysiloxane having a viscosity at 25xc2x0 C. of 1xc3x97105 cP or more, an inorganic filler and a treating agent are mixed in a high-speed mechanical shearing machine to provide a flowable particulate mixture in which each ingredient is present in a substantially uniform, finely dispersed state. The flowable particulate mixture is then fed at a constant feed rate into a kneading and extruding machine that has two screws rotating in the same direction and has a length to diameter ratio (L/D) of 25 to 50.
In Hamada et al., U.S. Pat. No. 5,409,978, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed at a temperature in the range of about 200xc2x0 C. to 300xc2x0 C. in a co-rotating continuous double screw extruder with L/D of about 25 to 50. The preconcentrate is then compounded and heat treated at 150xc2x0 C. to 300xc2x0 C. in a counter-rotating, double screw extruder. A useful L/D ratio for the extruder is in the range of about 5 to 15.
Highly vigorous first step processes for forming a composition of silicone polymer, filler and treating agent generate a product having a high volatiles content. The composition must be devolatilized in a second step to produce a useful product. Devolitilization is important for at least two reasons. First, devolitilization removes unreacted treating agents to xe2x80x9cquenchxe2x80x9d further reaction with filler silanol groups. Otherwise, further reaction can diminish filler reinforcement properties in the heat-vulcanizable silicone composition. Second, devolitilization removes surplus liquids that adversely affect rheological properties of the silicone composition. Residual volatiles in the filled silicone composition should not exceed the level of about 2% by weight of the final formulation. Preferably, residual volatiles in the filled silicone composition should not exceed the level of about 1%.
However, devolitilization is not complete in known two-step processes. There is a need for a process that continuously and consistently produces a devolatilized high viscosity filled silicone polymer composition from filler, additive and polymer. Further, there is a need for an improved continuous compounding process.
The invention provides a process that compounds high levels of inorganic filler, processing fluid and silicone polymer into homogeneous filled and devolatilized silicone compositions with requisite reinforcing properties and levels of volatiles. The method comprises mixing a filler, processing fluid and silicone polymer into a first compounding apparatus to produce a dispersed composition containing volatiles. The dispersed composition is then compounded in an extruder having an L/D ratio of greater than 50 to devolatilize the dispersed composition.
In another aspect, a process of compounding a filled silicone composition comprises forming a premix of filler and silicone polymer in a first mixer. The premix is then compounded with further filler and silicone polymer and added processing fluid in a first compounding apparatus to produce a dispersed composition containing volatiles. The dispersed composition is devolatilized in an extruder having an L/D ratio of greater than 50.
In still another aspect, a system is provided for compounding filled silicone compositions. The system comprises a first compounding apparatus and an extruder having an L/D ratio of greater than 50. The extruder is connected to the first compounding apparatus to continuously receive a dispersed composition containing volatiles.